One easy way is to buy a Lionel Powermaster and Cab-1. These are available most of the time on the used market (forum, auction sites) or the newer Legacy versions from dealers. The Powermaster allows you control track voltage and hit the whistle or horn with the remote control instead of using the transformer throttle and buttons. Together, that might set you back $100.
Note that this doesn't get you command control. That requires on-board electronics to control the engine, not just the track voltage.
If al you want to do is run the train with a remote, a Powermaster and Cab-1 remote is the least expensive and least complicated way to go. If you want full command control, then I'd recommend looking at ERR components from 3rd Rail. Upgrading an engine to full command control will set you back about $250 in parts. On top of that, you still need the Cab-1 or Legacy Cab-2 remote and a TMCC or Legacy command base instead of the PowerMaster. Even on the used market, you're looking at $200 for a Cab-1 and Command Base, more for the newer Legacy versions, and that's on top of the $250 for the ERR components.
For one engine, that's a big investment. If you're planning to expand your empire someday with more engines, this might not be a bad investment. But, if you just want one engine for around the Christmas tree, I'd suggest either (1) the Powercaster and Cab-1 or (2) a Lionchief or Lionchief Plus engine. With those, you don't need a command base, power master, or such to interface with the track. All the electronics you need are contained on board the engine. That keeps matters simple.
